Recently, with the evolution of a display technique, imaging of a high image quality has been demanded increasingly for a semiconductor imaging element. Space resolution is improved by increasing the number of imaging pixels and time resolution is improved by a high-speed imaging (high frame rate) operation. In addition, color reproducibility or a correction effect is increased by increasing the number of output bits per pixel (increasing a gradation).
In an analog/digital (A/D) conversion operation in a complementary metal oxide semiconductor (CMOS) image sensor, an A/D converter of a time integration type using a ramp waveform as a reference voltage has been widely used. The A/D converter of the time integration type is superior in linearity or a noise characteristic. A plurality of A/D converters are arranged for each pixel column and execute A/D conversions at the same time, so that an operation frequency of each A/D converter can be decreased. In addition, the A/D converters arranged for each pixel column can share a reference voltage generator with each other. Therefore, area/power efficiency is superior and compatibility with a CMOS image sensor is superior as compared with other A/D conversion methods.
However, in the case of the A/D conversion of the time integration type, a gradation increase and a conversion speed are in a trade-off relation. Therefore, it is necessary to sacrifice a frame rate to increase a bit gradation of output data.
Therefore, a method of connecting a plurality of comparators to one pixel vertical signal line, executing a comparison with a plurality of reference voltages at the same time, and improving a frame rate has been considered (for example, refer to Patent Document 1).
In addition, a method of dividing an input (intensity of light) for an imaging element into a plurality of sections, setting an inclination of an output, and operating a plurality of linear A/D converters in parallel at the same time, in nonlinear compression of a RAW image in which data is compressed without generating an unnatural gradation, has been considered.